In a conventional machine tool with a vertical spindle, the spindle is supported by a column in such a way that the spindle is movable along its axial direction or the Z-axis, and the column is either directly or indirectly supported on a base. In an exemplary arrangement shown in FIG. 21(a), an X-axis table 72 being movable in the X-axis direction is mounted on a base 71, and a column 73 serving as a Y-axis table being movable in the Y-axis direction is mounted on the X-axis table 72. A spindle head 74 is supported by the column 73 movably along the Z-axis, and a work bed 75 is connected to the base 71. In another arrangement shown in FIG. 21(b), an X-axis table 76 serving as a work bed and the column 73 serving as a Y-axis table for movement in the Y-axis direction are mounted on the base 71. The spindle head 74 is supported by the column 73 movably along the Z-axis direction. U.S. Pat. No. 5,439,431 typically discloses such type of machine tool, in which a spindle driven to move in Z-axis direction is mounted on a compound slide (corresponding to the column 73 in FIG. 21(b)) which is moved in X- and/or Y-axis direction. In yet another arrangement shown in FIG. 21(c), an X-axis table 78 serving as a work bed being movable in the X-axis direction is mounted on a Y-axis table 77 for movement in the Y-axis direction, which is disposed on the base 71. A column 79 of an inverted L shape is vertically mounted on the base 71, and a spindle head 80 having a built-in mechanism for movement in the Z-axis direction is mounted to the distal end of the column 79. In a still further example shown in FIG. 21(d), the Y-axis table 77 is mounted on the base 71 for movement in the Y-axis direction, and the X-axis table 78 serving as a work bed is mounted on the Y-axis table 78 for movement in the X-axis direction. A column 81 is vertically mounted on the base 71, and the spindle head 74 is supported by the column 81 movably along the Z-axis.
Other types of machine tools such as the one employing a gate-like column (not shown) or the one having its spindle arranged horizontally have substantially the same configuration as described above.
The spindle head is generally cantilevered by the column in any of the prior arrangements described above. In order to variably determine the positional relationship between the spindle head and the workpiece, the mechanisms that link the spindle head and the work piece are constructed to detour the space therebetween. To accomplish high-speed and highly precise machining, the machine tool must meet two requirements. Firstly, the materials constituting the base, table, column, and other members should be given high rigidity. Secondly, the play between moving parts of each member should be minimized. To meet these requirements, relatively larger members precisely constructed are necessary, causing the entire machine to be larger, heavier, more expensive, and to occupy a larger space in a factory line for installation.
Another problem is attributable to the detoured structure and the large scale of the entire machine. Thermal deflection caused by changes in temperature has greater influence on the mechanisms, adversely affecting the precision of machining.
In view of the foregoing, it is an object of the present invention to provide a machine tool which accomplishes a high degree of precision in machining with minimized size, weight, and cost.